Kirk McElhearn, writing on his blog about the difference between iTunes Match and iCloud Music Library:

Both match your iTunes library and store your purchases. Both allow you to access these files, and listen to them, on multiple devices. But with iTunes Match, when you download a matched or uploaded file, you get either the iTunes Store matched copy, or the copy that iTunes uploaded of your original file.

When you match and download files from iCloud Music Library, however, you get files with DRM; the same kind of files you get when you download files from Apple Music for offline listening.

The critical issue here is backing up your un-DRMed music files. Unlike iTunes Match, if you download matched un-DRMed files from iCloud Music Library, you’ll end up with a DRMed version of those files. To repeat this another way, if iCloud Music Library takes in a song file with no DRM and it has a DRM version of that same song, you’ll have no way to get back your original song file without DRM.

Back up your song library before you turn on iCloud Music Library, just to be safe.